The present invention relates to a method of printing a substrate with an inkjet printing device which comprises at least one print head provided with at least two rows of nozzles, wherein substantially fixed locations on the substrate, which locations form a regular field of pixel rows and pixel columns, are provided with ink drops image-wise, the resolution of the pixel columns being greater than the resolution of the rows of nozzles, so that p, where p is equal to the quotient of the resolution of the pixel columns and the resolution of the rows of nozzles, is an integer greater than or equal to 2. The method comprises a first printing stage in which a strip of pixel rows is provided with ink drops, whereafter the print head is displaced in a direction substantially parallel to the pixel columns, and a second printing stage in which the strip is provided with supplementary ink drops. The present invention also relates to a printing device suitable for the use of the method.
A printing method is known from U.S. Pat. 5,640,183. A known problem in inkjet printing devices is that deviations of individual nozzles may cause disturbing faults in the printed image. For example, a nozzle deviation may result in ink drops leaving the nozzle at the wrong angle (xe2x80x9cskew jetsxe2x80x9d), so that they occupy a different place on the substrate with respect to the center (the normal position) of the fixed locations (pixels), or result in ink drops with a deviant volume, so that too much or too little ink reaches the substrate. This method is used to mask the faults. The print heads used for the use of this method are provided with two rows of nozzles each having a resolution (number of nozzles per unit of length) equal to half the required printing resolution (number of fixed locations per unit of length) in a direction parallel to the pixel columns, and which together, by occupying an interlaced position with respect to one another, form a print head with the required printing resolution. Each row of nozzles of a print head is provided with a number of extra nozzles. When a strip of pixel rows of the substrate is printed by the known method, a series of successive nozzles is selected in the first printing stage from the set of the rows of nozzles of a print head, the number of nozzles in this series being equal to the total number of nozzles of the print head less the number of extra nozzles. If a print head is provided with two rows of 50 nozzles and 3 extra nozzles per row (so that the total number of nozzles is equal to 106), a series of 100 successive nozzles is selected with which a strip in a width of 100 adjoining pixel rows of the substrate is printed. After this first printing stage, a new series of 100 successive nozzles is selected from the available 106 nozzles of the print head. There are thus 7 different options for selecting a second series, i.e. the same series as used in the first printing stage and one of the other 6 possible series of 100 successive nozzles. A choice from these 7 options is made at random. After the choice has been made, the print head is displaced with respect to the substrate in a direction substantially parallel to the pixel columns over a distance corresponding to the selected second series of successive nozzles. The relevant strip is then provided with supplementary ink drops in the second printing stage. By printing each strip of pixel rows of the substrate with a plurality of sub-images, each of said images being printed by a series of successive nozzles chosen at random, any printing faults due to deviations of nozzles are distributed at random over the substrate so that they are less visible to the human eye.
A significant disadvantage of the known method is that as a result of the random choice there is an appreciable risk that a pixel row may be printed entirely with ink drops having the same fault, for example because they occupy a different position with respect to the normal position. Consequently, linear faults may occur in the image. The human eye is very sensitive to such linear faults and these faults are thus found to be disturbing in the printed image. A linear fault forms in any case if the first and second (and any following) series of successive nozzles are identical in the printing of a strip of pixel rows so that all the ink drops printed in one pixel row originate from one specific nozzle. It has also been found that within one print head there are many nozzles which have substantially the same deviations, i.e. they result in ink drops printed with the same fault. Consequently there is a considerable risk of linear faults when the known method is used.
Another disadvantage of the known method is that prior to the second and any following printing stages the substrate must be displaced very accurately over a distance which, depending on the choice of the second series of successive nozzles, varies at random with the width of 0, 1 or a number of pixel rows (a maximum of 6 in the above-described example). A shift of this kind is obtained by moving the paper with respect to the print head by means of a motor. These small shifts chosen at random mean that the paper transport must meet very stringent requirements with respect to accuracy.
Finally, the printing device productivity is reduced with respect to the maximum obtainable productivity since a number of nozzles in each row must be reserved as extra nozzles to make it possible for a random choice to be made for the second and any subsequent series of nozzles.
The object of the present invention is to obviate these disadvantages. To this end, a method has been developed wherein the print head is displaced over a distance being equal to the width of a number of pixel rows selected from the set:
xc2x1(i+kp)xe2x80x83xe2x80x83(formula 1)
where i is the set of integers greater than or equal to 1 and less than or equal to (pxe2x88x921), k is a natural number and p is equal to the quotient of the resolution of pixel columns and the resolution of rows of nozzles, wherein the resolution of the pixel columns is greater than the resolution of the rows of nozzles.
The present method is based on the realization that it is better to use the systematics of the deviations of the nozzles of the print head to mask printing faults than try to break through the same by a random choice as known from U.S. Pat. No. 5,640,183. The systematics governing the deviations of the nozzles may comprise a number of distinguishable forms of regularity.
First, it has been found that the deviation of the nozzle is substantially constant in time, irrespective of the intensity of the use of the nozzle. In other words, a nozzle will impart substantially the same fault to each drop ejected during the life of the print head. In addition, the deviations of the different nozzles within one row of a specific print head have been found to be not independent of one another in many types of print heads. It has been found that the deviation of an individual nozzle is substantially equal to the deviations of the adjoining nozzles within the same row. For example, if nozzle i in the first row of a print head has a deviation resulting in an ink drop originating from the same nozzle deviating from the normal position on the substrate by a distance of 20 xcexcm, then the ink drops originating from the nozzles ixe2x88x921 and i+1 will result in ink drops differing by about 20 xcexcm from the normal position. It has also been found that the deviations of the individual nozzles within one row frequently have a slow progression, so that not only the directly adjoining nozzles within one row have substantially the same deviations, but also the more distant nozzles. The deviation progress may also be said to be periodic, so that even nozzles very far away from one another have practically the same deviation. As a result of these forms of regularity, there may be many nozzles within one row which exhibit substantially the same deviations. The reason for this regularity is not entirely clear. One reason for the skew jets might be that such print heads are formed by stretching a foil formed with the nozzles over a base. Since this foil can never be stretched completely flat, there may be convexities therein (for example in the form of a corrugated pattern) so that ink drops are ejected from the nozzle at an angle deviation. Another reason might be the semi-continuous production process of such foils, resulting in periodic deviations.
The result of such regularities is that when the known method is used there is a great risk that a pixel row will be provided with ink drops all having the same fault, so that disturbing linear faults may occur in the image. By not using a random choice for the shift of a print head between the first and second (and any following) printing stages, but by making a selection from the set of shift distances given by formula 1, the ink drops printed on one pixel row are, at all times, prevented from all having the same fault. Consequently no disturbing linear faults will form in the image. It has also been found that the masking of faults due to incorrectly placed ink drops (skew jets) is better than when the known method is used.
In one preferred embodiment, the shift distance is equal to the width of a number of pixel rows where k is a natural number equal to or less than 20. The reason for this is that the masking of skew jets, the most common fault, is better the smaller the displacement distances. In another preferred embodiment, k is less than or equal to 10, so that the visible effects of any deviations of the nozzles can be masked even better. If k is smaller than or equal to 5, masking is further improved. The best masking of any deviation is finally obtained when k is equal to 0, so that the displacement is over a distance equal to the width of one pixel row.
For the use of the method according to the present invention, it is not essential for the second printing stage in which a number of pixel rows is provided with supplementary ink drops, to follow directly on the first printing stage. It is quite possible that first a number of strips of the substrate will be provided with a first series of ink drops, whereafter the pixel rows in each of these strips are provided with a supplementary second series of ink drops in a following printing stage. It is essential that the position occupied by the print head during the following printing stage, in order to provide a specific strip of pixel rows with the supplementary ink drops, should be selected in accordance with formula 1 with respect to the position occupied by the print head in printing the first series of ink drops on the pixel rows of said strip.
An arbitrary choice can be made from the set of shift distances given by formula 1. If an image is formed on a substrate by printing a number of strips, a different choice can be made for each of the strips. In principle, the choice for a shift distance for each of these strips can be made at random (from the set given by formula 1). However it has been found that the selection of one fixed shift distance for each of the strips also results in good masking of any printing faults. This is of course related to the systematics of the deviations of the nozzles. An important advantage of this is that in principle one fixed shift of a print head between each of the printing stages required to print a strip will be all that is required. A fixed shift means that the paper transport does not have to meet such strict requirements. Also, in principle, no extra nozzle need be added to a row, so that a print head can be used without loss of productivity.
It has surprisingly been found that the nozzle deviations may be subject to a third form of regularity. It has been found that the deviation patterns of corresponding rows of nozzles of different print heads produced in the same way may significantly correspond. If, for example, a 600 n.p.i. (nozzles per inch) print head consists of 3 rows of 200 nozzles, it appears that the deviations of the nozzles of the first row of this print head correspond substantially to the deviations of the first row of each following print head produced in the same way. The same naturally applies to all the second rows and all the third rows of these print heads. The result, using a number of print heads in a printing device, which print heads satisfy this third form of regularity, is that even linear faults may occur if the known method is used, when ink drops are printed in one pixel row originating from different print heads. By using the method according to the present invention for a set of print heads of this kind as well, i.e. by coordinating in accordance with formula 1 the relative positions of the two or more print heads used for printing a pixel row of the substrate in the different printing stages, linear faults are at all times prevented from forming in the image.
The present invention also relates to an inkjet printing device adapted for using the method according to the present invention. In a preferred embodiment, the print head comprises two rows of nozzles. By arranging for these rows to occupy an interlaced position, even using such rows of low resolution, it is possible to make a print head having a higher resolution, a double resolution in this case. In a further preferred embodiment, each row of nozzles of a print head of this kind has a resolution equal to half the resolution of the pixel columns.
In another preferred embodiment the printing device comprises at least two print heads. If a printing device contains a plurality of print heads, the present invention can be further utilised. This is apparent from the following. For use of the method according to the present invention, it is not essential for the first and second (and any following) printing stages to follow one another directly. This means that the different printing stages can also be performed by different print heads (which, if they were produced in a comparable manner, correspond significantly with respect to deviation pattern). It has also been found that the shift of the print head between the various printing stages may also be a fixed shift, for example always (i.e. for each strip of the substrate) equal to the width of one pixel row. This means that the method according to the invention can also be used by printing each sub-image in a pixel row with a separate print head, the mutual shift of the print heads already being embodied in the fixed arrangement of the print heads in the printing device scanning carriage. This means that the paper transport can be made very rugged, because it is no longer necessary to shift an individual print head with respect to the substrate over a distance equal to the width of one or a few pixel rows between each of the printing stages. A concomitant advantage of this printing device is that printing of the sub-images no longer requires separate printing stages for each sub-image, and instead, all the images can be printed in one printing stage. Of course given the correct arrangement of the various print heads, for example disposed next to one another in a scanning carriage with mutual positioning (in the direction parallel to the pixel columns) selected in accordance with formula 1, all the sub-images can each be printed with a separate print head in one printing stage, i.e. in one movement of the scanning carriage.
If a pixel row is printed with ink drops originating from two or more different print heads, then in a preferred embodiment the position of each following print head differs from the position of the print head used in the first printing stage by not more than the distance where k is an integer less than or equal to 20. In another preferred embodiment, these mutual positions do not differ by more than the distance where k is less than or equal to 10, so that the visible effects of any nozzle deviations can be masked even better. Masking is further improved if these mutual positions of pixel rows do not differ by more than the number where k is less than or equal to 5. The best masking of any deviations is finally obtained when k is equal to 0, so that the mutual positions do not differ by more than one pixel row.
Just as in the known method, using the method according to the present invention, a first sub-image is now printed with a specific print head on a strip of pixel rows of the substrate, whereafter the strip is provided with the other sub-images in one or more following printing stages. Assuming that the complete image can be printed by printing diluted sub-images which complement one another in three printing stages, if a print head used for the purpose is constructed from three rows of nozzles each having a resolution equal to one-third of the required printing resolution (p=3), the number of pixel rows over which the print head must be displaced after the first printing stage has taken place can be selected from
xe2x80x83xc2x1(i+kp)
where i=1 or i=2 (=pxe2x88x921) and k is a natural number=
xc2x1(1+k3),xc2x1(2+k3)=
xc2x1(1,4,7, . . . ),xc2x1(2,5,8, . . . )=
( . . . xe2x88x928,xe2x88x927,xe2x88x925,xe2x88x924,xe2x88x922,xe2x88x921,1,2,4,5,7,8, . . . )
An arbitrary choice can be made from this set, for example a shift distance equal to the width of 1 pixel row (i=1 and k=0). Applying formula 1 prevents the print head from being displaced between the first and each arbitrary following printing stage over a distance of xc2x1(0, 3, 6, . . . ) pixel rows, as a result of which the ink drops printed in one pixel row would originate from the same nozzle (shift distance=0) or a nozzle having substantially the same deviation (shift distance is 3, 6 etc.). This prevents any deviations of nozzles from being propagated in the direction of a pixel row.